1. Field of the Invention
The present invention relates to devices equalizing the states of charge of a plurality of cells that form an assembled battery, and assembled battery systems including the devices.
2. Description of Related Art
In recent years, an assembled battery has been widely used, for example an assembled battery including a plurality of lithium-ion secondary cells connected in series is used in a hybrid vehicle as a power source. Discharge power of an assembled battery is limited by a cell with the lowest state of charge (SOC) among a plurality of cells that form the assembled battery. Therefore, performance of the assembled battery decreases due to the variation of SOC of the plurality of cells that form the assembled battery. Accordingly, an equalization process is required to limit the variety of the SOCs of the plurality of cells that form an assembled battery within a certain range.
FIG. 9 shows a main part of a conventional state of charge equalizing device. A discharge circuit 51 comprising a resistor R and a switch SW connected in series is connected to both ends of each cell 4 that forms an assembled battery in parallel with respect to each cell. A voltage measurement circuit 52 which measures the voltage between both ends of each cell 4 (open-circuit voltage) is connected to both ends of each discharge circuit 51. Here, a plurality of discharge circuits 51 and a plurality of voltage measurement circuits 52 are mounted on one substrate, thereby configuring a voltage measurement unit 5.
In this state of charge equalizing device, the open-circuit voltage of each cell 4 is measured by each voltage measurement circuit 52, and an equalization target voltage is calculated based on the measured cell voltage. For example, the equalization target voltage is calculated by adding a predetermined value to the lowest value or an average value of the measured cell voltages. Thereafter, the discharge circuit 51 starts discharging each cell which has the open-circuit voltage higher than the equalization target voltage, and then the discharge circuit 51 stops discharging when the voltage measured by the voltage measurement circuit 52 reaches the equalization target voltage. Thus the SOCs of the plurality of cells that form the assembled battery are equalized.
However, in the conventional state of charge equalizing device described above, since the voltage measurement circuit 52 is connected to both ends of the discharge circuit 51 as shown in the figure, when each cell 4 is discharged, discharge current I1 and discharge current I2 flow in each discharging loop including the cell 4 and the discharge circuit 51 as shown by arrows in the figure, thereby causing a great voltage drop due to an internal resistance of the cell 4, a resistance component of and contact resistance between terminals existing between a cell terminal and a voltage detecting point, and a resistance component of a voltage detecting wire extending from the cell terminal to the voltage detecting point. As a result, the voltage measured by the voltage measurement circuit 52 becomes lower than the actual voltage.
As shown in FIG. 10, when the voltage of a cell B1 is V, the discharge current flowing in the discharging loop including the cell B1 and the discharge circuit 51 is I1, the internal resistance of the cell B1 is Rbat1, and the resistance component of the terminal, the contact resistance between terminals and the resistance component of the voltage detecting wire are RL1, a voltage VB1 which is measured by the voltage measurement circuit 52 connected to the cell B1 is described by a formula 1 below.VB1=V−I1(Rbat1+RL1)  Formula 1
Since the voltage measured by the voltage measurement circuit 52 is lower than the actual voltage as described above, even if the discharging of the cell 4 is ended at the time the voltage measured by the voltage measurement circuit 52 reaches the equalization target voltage, the actual cell voltage is higher than the equalization target voltage. Also, as shown in FIG. 11, since the length of a voltage detecting wire 41 extending from the plurality of cells 4 that form the assembled battery to a voltage measurement unit 5 varies from a cell to another, the amount of voltage drop due to the resistance component of the voltage detecting wire 41 varies from a cell to another. Thus, the difference between the voltage measured by the voltage measurement circuit and the actual cell voltage varies from a cell to another. Therefore, after the equalization process, the amount of actual cell voltage exceeding the equalization target voltage varies from a cell to another, thereby causing a problem of not being able to achieve high accuracy in the equalization process.
To solve this problem, it is possible to adopt a structure as shown in FIG. 12, in which terminals 54, 54 for connecting the voltage measurement circuit 52 to both ends of the cell 4 are provided separately from terminals 53, 53 for connecting the discharge circuit 51 to both ends of the cell 4, and the voltage measurement circuit 52 is connected to the terminals 54, 54. According to this structure, it is possible to reduce the amount of voltage drop due to the resistance component of and contact resistance between the terminals existing between the cell terminal and the voltage detecting point, and the resistance component of the voltage detecting wire extending from the cell terminal to the voltage detecting point.
Also, there has been proposed a method of calculating the amount of voltage drop due to the internal resistance of the cell and correcting the equalization target voltage based on the calculated amount of voltage drop.
However, in the structure in which the voltage measurement circuit 52 is connected to both ends of the cell 4 as shown in FIG. 12, the number of terminals connecting to both ends of the cell and the number of connecting wires extending from the cell 4 to the voltage measurement unit 5 double, thereby causing a problem of making the structure complicated and increasing the manufacturing cost. In addition, in the structure described above, even though the amount of voltage drop due to the resistance component of the terminal, the contact resistance between the terminals and the resistance component of the voltage detecting wire reduces, the amount of voltage drop due to the internal resistance of the cell 4 is still large, thereby causing a problem of not being able to perform the equalization process with high accuracy.
Also in the conventional method of calculating the amount of voltage drop due to the internal resistance of the cell and correcting the equalization target voltage based on the calculated amount of voltage drop, there is a problem of not being able to perform the equalization process with high accuracy. This is because the voltage drop, which is the cause of the difference between the actual cell voltage and the voltage measured by the voltage measurement circuit, includes not only the voltage drop due to the internal resistance of the cell, but also the one due to the resistance component of the terminal, the contact resistance between the terminals and the resistance component of the voltage detecting wire as described above, and therefore the correction of the equalization target voltage based on the voltage drop due to the internal resistance of the cell is inadequate.